A turbocharger improves the efficiency of an internal combustion engine by increasing the pressure and density of the intake air. At the outlet of the engine, an engine's exhaust gases are directed to a turbine wheel translating exhaust energy into rotational mechanical energy of a shaft. The shaft couples the turbine to a compressor disposed in the intake flow of the engine. The compressor increases the pressure and density of the intake flow so that the air-fuel mixture is more combustible. The increase of the mass of air creates more power and torque when the piston is forced downward by the resulting explosion. This process results in a boost to overall engine power.
Turbochargers are required to operate over a wide range of engine speeds and loads. Systems have been developed to precisely control the boost provided by the turbocharger by controlling the exhaust gas provided to the turbine of the turbocharger. The boost pressure is the main control variable. There are several ways to control boost pressure. In one system, a waste gate valve is controlled to bypass a portion of the exhaust thereby controlling the flow rate through the turbine and, thus, the amount of work transferred through the shaft to the compressor. These types of turbochargers are called waste gate turbochargers.
Another control mechanism for controlling the amount of boost provided by a turbocharger includes variable nozzle turbines (VNT) that effectively vary the geometry of the turbine nozzle. These turbochargers are called variable nozzle turbine turbochargers. Such VNT mechanisms include multiple movable aerodynamic blades in the nozzle, or pistons with or without vanes comprising one wall of the nozzle, which are axially movable with respect to a fixed nozzle wall. Control of these mechanisms varies depending on application and can include pneumatic, electromechanical, hydraulic, and electro-hydraulic actuation systems. The control of a turbocharger is complicated by the inherent lag in the engine exhaust system and the transient response times of the mechanical elements of the variable-geometry mechanism.
As stated, the goal of the control system is to maintain the pressure at the inlet of the engine (boost pressure) by manipulating either the waste gate or VNT position. Control of these inputs with respect to boost pressure is non-linear. Additional non-linearities are added by the presence of saturation and backlash in the mechanical actuators and other mechanical parts.
An exhaust gas recirculation valve (EGR) is another variable to the system which is also present in many systems. An EGR connects the exhaust and inlet manifolds and recirculates a portion of the exhaust gas back into the inlet of the engine. The EGR is controlled for purposes of state and federal emission regulations compliance. In this case, the control variable is air flow (mass flow rate) through the engine. Due to the stringent emission regulations, the VNT and EGR have to be controlled at the same time. Therefore, two engine variables must be controlled through VNT and EGR, boost pressure and air flow, respectively.
Air flow and boost pressure closely effect one another. On one hand, a variation of air flow through the turbine due to an EGR change affects the turbine power and thus the boost pressure, therefore requiring quick compensation by the VNT control. On the other hand, a variation in the VNT position affects both boost pressure and engine back pressure, which ultimately changes EGR flow. Therefore any change in VNT position requires compensation by EGR control, and vice versa.
From system identification tests, the response of a turbocharger system may be modeled by a linear transfer function that varies with engine operating conditions, for example, a second order transfer function:             Y      ⁡              (        s        )                    U      ⁡              (        s        )              =      K                  s        2            +              2        ⁢        ξ        ⁢                                   ⁢        ω        ⁢                                   ⁢        s            +              ω        2            
Where the input U(s) is the s-domain (Laplace transform) pulse width modulated command signal provided to the VNT actuator or the waste gate actuator; Y(s) is the s-domain boost pressure; K is the static gain; ω is the natural system pulsation, and ξ is the damping coefficient. The gain, pulsation, and damping also vary as a function of engine state, e.g., engine speed and load.
Previous control strategies included PID (proportional-integral-derivative) controllers with gain schedules based on engine speed and load. However, these approaches require extensive calibration to determine proper PID gains at many different engine states. Other approaches, such as mu-synthesis controllers have also required extensive calibration procedures. Therefore it is desirable to develop a more robust controller, which would allow the usage of a reduced set of universal gains, leading to improved calibration.